This invention relates to energy absorbers used in conjunction with large structures to reduce the influence of externally induced motion on such structures.
Cyclic shear energy absorbing devices are known which employ the cyclic plastic deformation of certain materials beyond the elastic limit for the absorption of kinetic energy. Such absorbing devices are typically interposed between a building support member and a base member, or between two structural support members, in order to convert portions of the kinetic energy into heat in the absorbing material and thus reduce the motion imparted to the structure by externally induced forces, such as an earthquake or high winds. U.S. Pat. No. 4,117,637 issued Oct. 3, 1978, to Robinson for "Cyclic Shear Energy Absorber", the disclosure of which is hereby incorporated by reference, illustrates several geometrical configurations of the basic cyclic shear energy absorber device. The basic device includes a pair of spaced coupling members, typically plates, each one of which is designed to be coupled to an individual structural member. When used in a building environment, for example, one of the coupling members is configured to be attached to a support piling, while the other coupling member is configured to be attached to a support pillar, beam or the like. Arranged between the two coupling members is a solid plastically cyclically deformable mass of material, typically lead, which provides the energy absorption function. Some configurations of this type of device further include an additional resilient pad structure which surrounds the energy absorbing mass and provides resilient vertical support between the two coupling members, usually by means of a sandwich comprising alternate layers of a resilient material (e.g. rubber) and a stiffener material (e.g. steel, aluminum or the like).
In use, when externally induced forces result in relative lateral motion between the two coupling members, the solid energy absorbing mass is cycled beyond its elastic limit, converting some of the energy into heat and storing the remaining energy when the mass is in the deformed state, the latter acting as a driving force which tends to return the material to its original mechanical properties. As a consequence, the energy transmitted to or through the structure is converted into heat rather than being applied in a destructive fashion to the building. Consequently, structures incorporating such absorbers have a higher safety factor than those relying on the ductile behavior of structural members to dissipate energy (which will be damaged by a severe earthquake and will be difficult to repair or replace), and those using rubber dampers, (which function in a spring-like fashion and dissipate only small amounts of externally imparted energy).
While cyclic energy absorbers of the above type have been found to function well in many applications, in some applications premature degradation of the energy absorbing mass after a small number of oscillations has been observed.
This is due to a lack of confinement about the absorber mass which is free to elongate in a direction normal to that of the imposed deformation and thereby reducing its effectiveness as an energy absorber. Even in those applications in which the energy absorbing lead core is surrounded by a resilient support pad having sandwich construction, the degree of confinement is dependent on the magnitude of the vertical load, the elastomer hardness and the thickness of the individual layers of elastomer. Specifically, the performance of the lead core may degrade if the vertical load is less than 0.4 times the rated load of the support pad at 0.5 shear strain for an elastomer hardness index between 50 and 55 and an elastomer layer thickness of 0.5 inches. It is the object of this invention to provide an improved cyclic shear energy absorber in which this diminution in performance is eliminated.